Surface treatment of aluminum base alloys and resulting product



United States Patent 3,180,806 SURFACE TREATMENT OF ALUMINUM BASE ALLOYS AND RESULTING PRODUCT Ernest H. Hollingsworth, New Kensiugton, Pa., assignor to Aluminum Company of America, Pittsburgh, Pa., a corporation of Pennsylvania No Drawing. Filed July 3, 1961, Ser. No. 121,304 21 (Claims. (Ci. 204l29) This invention relates in general to the manufacture of precipitation hardenable aluminum base alloy products which are responsive to coloration by anodic treatment.

The invention is more specifically addressed to the-pro vision of hot worked and quenched products of an aluminum base alloy which exhibit increased strength when precipitation hardened and yet possesses the ability to acquire a pleasing uniform surface coloration when anodically treated by virtue of the combination of alloy constituents in dissolved and in precipitated forms. Surface coloration by anodic treatment refers to the result obtained by treating the products in an acidic electrolyte, the product being the anode, whereby a uniform, integrally bonded colored anodic film is formed.

it is known that non-heat treatable and non-precipitation hardening binary aluminum-chromium alloys containing from 0.2 to 0.5% by weight of chromium, where the chromium is in a state of solid solution, can yield a gold colored product when subjected to a conventional anodic treatment; To attain this result the chromium must not only be in solid solution but the total iron and silicon impurities must not exceed 0.35% by weight. To increase the strength of the alloy it has been proposed that 0.4 to 1.2% magnesium be incorporated therein but even with this addition the alloys do not possess a high enough strength, when extruded, to meet some of the demands imposed on structures, especially in the architectural field.

The present invention meets the need for aluminum base alloy products which are not only responsive to a silicide upon the coloration attributable to the presence 7 of chromium.

It is therefore an object of the present invention to pro vide solution heat treated products of the aluminum-magnesium silicide-chromium type of alloys which, either with or without subsequent precipitationhardening, respond to anodic treatments and yield a colored anodic film. A particular object is to provide an extruded product of an aluminum-magnesium silicide-chromium alloy' which in the precipitation hardened condition yields an integrally bonded uniform surface coloration when anodically treated. A further object is toprovide a method of treating an aluminum-magnesium silicide-chromiurn alloy prior to hot working and subsequent thereto in a manner which will establish a metallurgical structure that will yield a pleasing uniform surfacecoloration when subjected to an anodic oxidation treatment. b

Other objects and advantages of the invention will become apparent from the following-description and examples. g

An intensive study and investigation of the response to anodic treatment of solution heat treated and precipitation hardenable aluminum-magnesium silicide-chromiurn alloy products has shown that following the conventional practices used in preheating aluminum base alloy ingots or other stock adapted to be worked, hot working the preheated stock, solution heat treating and precipitation hardening the work alloy, the final product will have little, if any, of the desired uniform integral color. It has been ascertained that the coloration of the anodic film is determined by the existence of the chromium in solid solution, but in the presence of magnesium and silicon in certain proportions the solubility of chromium is greatly reduced with a resultant loss of color. Magnesium and silicon in the proportions found in the intermetallic compound, magnesium silicide, appear to cause the precipitation of a substantial portion of any chromium that may have been in solid solution in the cast ingot. Moreover, the usual solution heat treatment given to aluminum-magnesium silicide type of alloys to increase their strength causes a precipitation of chromium in aluminummagnesium silicide-chromium alloys with a consequent loss in color when subjected to an anodic oxidation treatment.

My invention is predicated upon the discovery that a uniform integrally bonded colored anodic film can be produced on products of an aluminum base alloy containing both chromium and elements or constituents which in certain proportions destroy or greatly impair the coloration efrect of chromium. In particular, I have found that an aluminum-magnesium silicide-chromium alloy can be employed in the manufacture of wrought products that are subjected to a thermaltreatment to improve their physical properties and still obtain a pleasing uniform surface coloration when anodically treated. The product not only possesses a desirable uniform color but it also exhibits a level of tensile strength which meets demands for load-bearing structural members in building or other installations. The product can also be employed where color is the principal desideratum and strength is of secondary importance such as in a composite body which includes both colored and non-colored components.

To obtain the product referred to above, the chemical composition, thermal treatment of the metal body and the anodic treatment must be controlled. The alloy must contain as its essential components aluminum, magnesium, silicon, and chromium, the magnesium and silicon being present in the proportions necessary to form 0.75 to 2% by weight of the intermetallic compound magnesium silicide (Mg Si) and a chromium content of 0.20 to 0.40%. A slight excess of magnesium or silicon above the amount requried to form the compound is usually present since the exact proportion necessary to form Mg Si is seldom attained in commercial operations. Generally, it is preferred to provide an excess of magnesium and thereby obtain the complete combination with all the silicon. Thus, the magnesium content may exceed that required to form Mg si by as much as 0.5% by weight. The silicon content may likewise exceed the proportion found in Mg Si but only up to 0.3% by weight. in terms of total magnesium and silicon contents this means that the magnesium may vary between 0.48% and 1.77% by weight and the silicon can be present in amounts between 0.27'

and 1.03% by weight. The foregoing minimum limit on the amount of Mg si should be observed in order to achieve the desired strength in the precipitation hardened prdouct while 2% by weight is slightly above the upper limit of solid solubility of the compound in aluminum.

Additional amounts of the compound do not increase the benefit gained by the precipitation hardening treatment. In my preferred practice I use from 0.90 to 1-20% by Weight of the intermetallic compound. The limits on the chromium content are necessary in order to obtain the desiredcolor effect in the anodic film. In my preferred practice the minimum amount of chromium should be 3 0.25% by weight in order to provide as nearly a saturated solid solution of that element in aluminum as possible when the alloy has received the proper initial thermal treatment.

With respect to other elements which may be present in the alloy, the iron impurity should not exceed 0.50%, while the upper limit for copper, manganese and zinc impurities should be 0.10% each. To achieve the best results I prefer to limit the iron to a maximum of 0.25% and the other named elements to a maximum of 0.05% each. If it is desired to refine the grain size of the extruded product, one or more elements of the following group may be employed in the indicated proportions: 0.001 to 0.10% boron, 0.01 to 0.25% titanium, 0.01 to 0.25% zirconium and 0.01 to 0.25% molybdenum, the total amount of these elements not to exceed 0.30%.

The thermal treatment and metal working practices of the invention employed in the production of articles of the above-described alloy are critical. The ingots or previously worked bodies from which the Wrought products are to be made should receive an initial high temperature treatment consisting of holding the ingots or wrought bodies within the temperature range of 1075 to 1130 F., and preferably 1100 to 1130 F. for a period sufficient to cause substantially complete solution and uniform distribution of the magnesium, silicon and chromium after which they are removed from the heating zone, i.e. the heating furnace, and cooled to at least the hot working temperature and usually to room temperature. The

cooling rate is not important, cooling in air being quite a satisfactory since there is very little, if any, precipitation of chromium under these conditions. To accomplish the desired solution and homogenization the thermal treatment should usually extend over a period of from 4 to 48 hours depending upon the size of the alloy body being treated. A maximum solution and uniform distribution of the soluble constituents is essential in the ingot, or other form of stock for working, to the success of the coloration process and properties of the final product. The uniform distribution of the dissolved alloy components is important in obtaining a uniform coloration of the wrought product. In the absence of the thermal treatment, the surface coloration tends to be non-uniform in some cases, especially if the alloy is severely worked. In view of the solution effected by the treatment it may be referred to as a solution heat treatment.

If the stock treated in the foregoing manner is cooled to room temperature, as is generally the most convenient practice, the ingots or wrought bodies are rapidly reheated to a suitable hot working temperature but not held at that temperature or within hot working temperature range for any prescribed length of time. The hot working temperature range lies between 700 and 1050 F. but for extrusion the temperature range should be above 750 F. Rapid reheating in this manner does not adversely affect the state of solid solution of chromium established by the initial thermal treatment. In referring to hot working it is to be understood that this includes such operations as extrusion, rolling, forging, pressing and the like, however, products made by the extrusion process lend themselves most readily to the treatments described herein.

The hot Worked product should be quenched with an air blast or a water spray or submerged in a liquid quenching medium, such as water, or it may even be allowed to cool naturally in air, if the cross-sectional dimensions of the product are small enough, but the cooling must be rapid, in any event, and therefore it is referred to as being a quench. It is the purpose of the quench, in any case, to retain as much of the magnesium and silicon in solution as possible and thereby increase the strength and hardness of the product as compared to the non-heat treated product and to establish a favorable condition for any subsequent precipitation hardening treatment. The chromium is also retained in solid solution along with the magnesium and silicon. The product can be used in the as-quenched condition, if desired, although the strength and color are not at maximum values.

To achieve a higher strength and improvement in color in the final product the quenched article should be precipitation hardened by heating it to a temperature between 300 and 475 F., usually for a period of from 1 to 24 hours. This induces precipitation of the Mg Si constituent but not chromium. The choice of temperature and length of treatment will be determined by the level of strength and the color which are desired. Extruded products, for example, which are treated at 400 to 475 F. for a period of 1 :to 6 hours can develop a tensile strength on the order of 27,000 p.s.i., a yield strength of 21,000 p.s.i. and an elongation of 12% and permit the formation of a wider range of colors than where the hardening is done at lower temperatures. The strength, in any case, is greater than that of the same alloy devoid of silicon in an amount sufiicient to form at least 0.75% of Mg Si. The precipitation treatment, it should be pointed out, not only does not interfere with the coloration imparted by the subsequent anodic treatment but generally improves it. The provision of a precipitation hardened product, such as one treated at 400 to 475 F. that can develop colors from gold to black, offers many advantages over products that have but limited range of color possibilities.

If a further improvement in strength is desired, the product can be cold worked, either before or after the precipitation hardening treatment. The amount of cold working can vary within relatively wide limits, especially before the precipitation hardening. Reductions in crosssection of up to 50% have been made without destroying the ability of the product to be subsequently colored by anodic oxidation. The cold work may affect the diffuseness and apparent color of the product to a small extent.

The final step in the process consists of anodically oxidizing the as-quenched or the precipitation hardened product in a suitable acidic electrolyte. The choice of electrolyte and magnitude of the current affect the coloration obtained. Thus, in a 15% sulfuric acid aqueous solution maintained at 65 to 75 F. and with a current density of 12 amperes per square foot, a uniform golden color is produced. On the other hand, the same electrolyte when used at a temperature of 25 to 28 F. and at a current density of 36 amperes per square foot develops a gray-brown, bronze or near-black color depending on the length of treatment. Still other colors, including black, can be obtained by treatment in other electrolytes, such as electrolytes containing oxalic or sulfosalicylic acids or mixtures of these acids with sulfuric acid at suitable current densities. These electrolytes, as Well as those of inorganic acids, such as phosphoric and chromic are known in the anodic treatment art and can be used, but obviously a selection must be made to develop a particular color.

The coloration developed by the anodic treatment can be modified by treatment of the surface of the product prior to anodic oxidation. Thus, the surface can be chemically brightened by exposure to a solution of phosphoric and nitric acids. The surface can also be brightened by an electrochemical means. Mechanical treatments such as bufing, polishing or sand blasting, can be used to alter the texture of the surface which in turn can modify the appearance of the final product. Chemical etching treatments offer still another method of moditying the texture of the surface. The mechanical and chemical treatments can be combined, if desired.

It is usually desirable to seal the anodic coatings in conventional manner, for example, by immersing the.

coated product in water at 208 to 212 F. for a period of 5 to 20 minutes or in hot aqueous solutions of certain nickel salts.

It is to be understood that while the primary object of the invention is to provide a surface coloration by arlodic treatment, these colors can be modified by impregnation of the oxide film with known substances of either an organic or inorganic character. Such modified colored coatings, of course, are subject to the limitations in service which are recognized in the art.

The benefits gained by my invention are illustrated by the following examples of the treatment of ingots and extrusions made therefrom composed of an alloy having a nominal composition by weight of 0.65% magnesium, 0.35% silicon, 0.33% chromium and balance aluminum and impurities. The ingots were cast by the direct chill continuous casting process in the form of cylinders 9 inches in diameter. The ingots were cut in sections for subsequent extrusion and heated for 16 hours at 1100 to 1125 F., following which they were removed from the furnace and cooled in air to room temperature. The sections were rapidly heated to 980 F. by electrical induction means and. extruded into molding strip Me" in thickness and 4 inches in width. The extruded product was quenched in an air blast as it left'the extrusion die. The tensile properties of a portion of the extrusions were etermined in this condition and another portion subjected to an anodic treatment in a sulfuric acid solution maintained at 70 F. with a current density of 12 ampercs per square foot. The treatment extended over a period of 60 minutes and produced an oxide film 0.9 mil in thickness. Another group of the as-quenched extruded products was precipitation hardened by heating them for one hour at 460 to 475 F. The tensile properties of'this group were determined and a portion was anodicaily treated in the manner described above. Still another group was precipitation hardened by heating the as-quenched extrusions for three hours at 375 to 390. The tensile properties of specimens from this group were determinedand a portion anodically treated according to the practice referred to above.

In each case the anodically coated products acquired a uniform yellow color but to ascertain the degree of color attained specimens were tested in a photoelectric trisitimulus co lorimeter of the kind described in the US. National Bureau of Standards Circular No. C429 by R. 8. Hunter. The yellowness measurements are expressed in terms of percent, the higher the value the more intense the color.

The tensile properties of the various extruded products and the percent yellowness developed by the anodic treatment are given in Table I below.

TABLE I Tensile properties and yellownessof extruded products lit will he observed that the precipitation hardening treatment raised the tensile and yield strengths but did not adversely aliect the intensity of color. It is also significant that the products which were hardened at the higher temperature acquired more color than those hardened at the lower temperature.

The eitect of cold working the as-quenched extruded products in respect to tensile properties and coloration is illustrated in the following examples. Samples of the its-quenched material were divided into two groups and one was cold rolled with :a reduction in thickness of 20% while the second one was cold rolled 40%. A portion of each group was used for-tensile property tests' and a second portion was anodically treated as described above. Some of the cold rolled material from each group was precipitation hardened by heating one hour at 460 to 475 F. Tensile tests were made on part of the samples and another part was anodically treated in the same manner as in the preceding examples. The yellowness of the variously treated samples was determined by a coloiimeter in the same manner as that set forth to form the magnesium silicide con1ponent.'

t is apparent that the cold working served to increase the tensile and yield strengths over the values of the as-quenched product but the color was reduced. Upon precipitation hardening, however, the color was restored but the strength values were lowered although not to the level of the samples which had been precipitation hardened at 460 to 475 F. as given in Table I.

In comparison with the foregoing I have observed that where the ingots have received a preliminary treatment at temperatures below 1000 F. before extrusion or the extruded products have received a conventional solution eat treatment, or both, the color is greatly reduced.

A black color, instead of yellow color, was developed on an extruded product of the above alloy which was precipitation hardened 1 hour at 460 to 475 F. and anodically treated in a 15% sulfuric acid solution maintained at a temperature of 25 to 28 F. and with a current density of 36 'amperes per square foot where the treatment extended over a period of 60 minutes.

A bronze color was produced on material precipitation hardened in the same manner by treating it in a solution of 7 grams per liter of sulfuric acid and 100 grams per liter of sulfosalicylic acid for a period of 30 minutes, at a temperature of F. and at a current density of 24 amperes per square foot.

Having thus described my invention and certain embodiments thereof, I claim:

1. The method or" making high strength anodical'ly oxidized wrought aluminum base alloy products comprising providing a body of an aluminum base alloy consisting essentially of aluminum, magnesium, silicon and 0.20 to 0.40% by weight of chromium, said magnesium andsili-v con being present in the proportions to form 0.75 to 2% by weight of magnesium silicide, heating said body to a temperature between 1075 and 1130 F. and holding it within said temperature range for 1a sutliciently long eriod to produce substantially complete solution and tuiiform distribution of the magnesium, silicon and chromium, removing the body from the heating zone, cooling said body to at least the hot working temperature, hot working said body within the hot working temperature range, immediately quenching said hot worked product, and :anodically oxidizing said product in an acidic electrolyte whereby an integrally bonded uniformly colored oxide film is developed.

2. The method according to claim 1 wherein the alloy contains up to 0.5% of magnesium in excess of that required to form the magnesium silicide component.

3. The method according to claim 1 wherein the alloy contains up to 0.3% of silicon in excess or that required 4. The method according to claim 1 wherein'theheat treatment is carried out at 1100 to 1130 F.

5. The method accord ng toclalrn 1 wherein the heat treatment extends over a period of 4 to 48 hours.

6. The method of making high strength and anodically oxidized wrought aluminum base alloy products comprising providing a body of an aluminum base alloy consisting essentially of aluminum, magnesium, silicon and 0.20 to 0.40% by weight of chromium, said magnesium and silicon being present in the proportions to form 0.75 to 2% by weight of magnesium silicide, heating said body to a temperature between 1075 and 1130 F. and holding it within said temperature range "for a sulficient length of time to obtain substantially complete solution and uniform distribution of the magnesium, silicon and chromium, removing the body from the heating zone, cooling said body to at least the hot working temperature within the range of 700 to 850 F., hot working said body within said temperature range, quenching said hot worked product, precipitation hardening said quenched product by heating it to a temperature within the range of 300 to 475 F. for a period of 1 to 24 hours, and anodically oxidizing said product in an acidic electrolyte whereby an integrally bonded uniformly colored oxide film is developed.

7. The method according to claim 6 wherein the alloy contains up to 0.5% of magnesium in excess of that required to form the magnesium silicide component.

8. The method according to claim 6 wherein the alloy contains up to 0.3% of silicon in excess of that required to form the magnesium silicide component.

9. The method according to claim 6 wherein the alloy contains at least one of the grain refining elements of the group composed of 0.001 to 0.10% boron, 0.01 to 0.25% titanium, 0.01 to 0.25% zirconium and 0.01 to 0.25% molybdenum, the total amount of said elements not exceeding 0.30%.

10. The method of claim 6 wherein the alloy contains magnesium and silicon in the proportions to form from 0.90 to 1.20% of Mg Si.

11. The method according to claim 6 wherein the said precipitation hardening is accomplished by heating to a temperature of 400 to 475 for a period of 1 to 6 hours.

12. The method according to claim 6 wherein the hot worked and quenched product is cold worked prior to the precipitation hardening treatment.

13. The method according to claim 6 wherein the hot worked and quenched product is cold worked subsequent to the precipitation hardening treatment.

14. The method of making high strength and anodically oxidized wrought aluminum base alloy products comprising providing a body of an aluminum base alloy consisting essentially of aluminum, magnesium, silicon and 0.20 to 0.40% by weight of chromium, said magnesium and silicon being present in the proportions to form 0.75 to 2% by weight of magnesium silicide, heating said body to a temperature between 1075 and 1130 F. and holding it within said temperature range for a sufficient length of time to obtain substantially complete solution and uniform distribution of the magnesium, silicon and chromium, removing the body from the heating zone, cooling said body to room temperature and rapidly reheating it to a second temperature range of from 750 F. to 1050 F., extruding said body within said second temperature range, quenching said hot extruded product, precipitation hardening said quenched product by heating it to a temperature within the range of 300 to 475 F. for a period of l to 24 hours, and anodically oxidizing said product in an acidic electrolyte whereby an integrally bonded uniformly colored oxide film is developed.

15. An anodically treated product composed of a body of aluminum base alloy consisting essentially of aluminum, magnesium, silicon and 0.20 to 0.40% by weight of chromium, said magnesium and silicon being present in the proportions to form 0.75 to 2.0% magnesium silicide, said product having an internal structure produced by heating said body to a temperature between 1075 and 1130 F. and holding it within said temperature range for a sufiiciently long period to produce substantially complete solution and uniform distribution of the magnesium, silicon and chromium, removing the body from the heating zone, cooling said body to at least the hot working temperature, hot working said body within the hot working temperature range, immediately quenching the hot worked product, precipitation hardening said quenched product by heating it to a temperature within the range of 300 to 475 F. for a period of 1 to 24 hours, and anodically oxidizing said product in an acid electrolyte, said product being characterized by having a substantial portion of the chromium in solution and an integrally bonded uniformly colored anodic film.

16. A product according to claim 15 wherein the alloy contains up to 0.5% of magnesium in excess of that required to form the magnesium silicide component.

17. A product according to claim 15 wherein the alloy contains up to 0.3% of silicon in excess of that required to form the magnesium silicide component.

18. A product according to claim 15 wherein the alloy contains at least one grain refining element of the group composed of 0.001 to 0.10% boron, 0.01 to 0.25% titanium, 0.01 to 0.25% zirconium and 0.01 to 0.25% molybdenum, the total amount of said elements not exceeding 0.30%.

19. A product according to claim 15 wherein the alloy contains magnesium and silicon in the proportions to form from 0.90 to 1.20% of Mg Si.

20. A product according to claim 15 wherein the hot worked product is an extrusion.

21. An ano-dically treated product composed of a body 35 of aluminum base alloy consisting essentially of aluminum, magnesium, silicon and 0.20 to 0.40% by weight of chromium, said magnesium and silicon being present in the proportions to form 0.75 to 2.0% magnesium silicide, said product having an internal structure produced by heating said body to a temperature between 1075 and 1130 F. and holding it within said temperature range for a sufficiently long period to produce substantially complete solution and uniform distribution of the magnesium, silicon and chromium, removing the body from the heating zone, cooling said body to at least the hot working temperature, hot working said body within the hot working temperature range, immediately quenching the hot worked product, and anodically oxidizing said product in an acid electrolyte, said product being characterized by having a substantial portion of the chromium in. solution and an integrally bonded uniformly colored anodic film.

References Cited by the Examiner UNITED STATES PATENTS OTHER REFERENCES Chadwick: J. Inst. Metals; 82: 75-80 (1953).

Chem. Abstracts, 52: 18128e (1958).

Budgen: Aluminum and Its Alloys, Pitman and Sons Ltd., 1947, pages 139-465, 248256, and 323-331.

JOHN H. MACK, Primary Examiner.

N R. ECK, MURRAY TILLMAN, Examiners. 

1. THE METHOD OF MAKING HIGH STRENGTH ANODICALLY OXIDIZED WROUGHT ALUMINUM BASE ALLOY PRODUCTS COMPRISING PROVIDING A BODY OF AN ALUMINUM BASE ALLOY CONSISTING ESSENTIALLY OF ALUMINUM, MAGNESIUM, SILICON AND 0.20 TO 0.40% BY WEIGHT OF CHROMIUM, SAID MAGNESIUM AND SILICON BEING PRESENT IN THE PROPORTIONS TO FORM 0.75 TO 2% BY WEIGHT OF MAGNESIUM SILICIDE, HEATING SAID BODY TO A TEMPERATURE BETWEEN 1075 AND 1130*F. AND HOLDING IT WITHIN SAID TEMPERATURE RANGE FOR A SUFFICIENTLY LONG PERIOD TO PRODUCE SUBSTANTIALLY COMPLETE SOLUTION AND UNIFORM DISTRIBUTION OF THE MAGNESIUM, SILICON AND CHROMIUM, REMOVING THE BODY FROM THE HEATING ZONE, COOLING SAID BODY TO AT LEAST THE HOT WORKING TEMPERATURE, HOT WORKING SAID BODY WITHIN THE HOT WORKING TEMPERATURE RANGE, IMMEDIATELY QUENCHING SAID HOT WORKED PRODUCT, AND ANODICALLY OXIDIZING SAID PRODUCT IN AN ACIDIC ELECTROLYTE WHEREBY AN INTEGRALLY BONDED UNIFORMLY COLORED OXIDE FILM IS DEVELOPED. 